Hello Prada,

You use the plate element in place as an analogy for your concrete foundation which is incompressible or always known to have insignificant compression due to the high rigidity index. Therefore what Plaxis will give you is the displacement due to soil.

I mean displacement due to soil compression.

In PLAXIS we can not define our own interface and study the interface behaviour between two element. In PLAXIS interface automatically generated between structural and soil elements. Also, the interface behaviour can be controlled by using the tool bar interface. Interface tool bar is there where do you need to define the model for soil ex. Mohr Coulomb with assigning model parameters for soil and interface. Further details, I recommend to you to read PLAXIS tutorial manual.

The relative displacement between the foundation and soil can read from the results table by knowing the local coordinates for the foundation and soil. Hope i have answered to your question.